Some Crystallo-chemical Relations of Nephelines and Feldspars on Stjernöy, North Norway
Chemical, X-ray, and optical data are given for coexisting nephelines and potassium feldspars separated from a nepheline syenite, and of nepheline and albite from pegmatites. Li, Rb, Cs (detected only in one nepheline), Ba, and Sr were determined, in addition to the major elements. (Pb and Tl were n...
| Published in: | Journal of Petrology |
|---|---|
| Main Author: | |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1966
|
| Subjects: | |
| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/7/1/95 https://doi.org/10.1093/petrology/7.1.95 |
| Summary: | Chemical, X-ray, and optical data are given for coexisting nephelines and potassium feldspars separated from a nepheline syenite, and of nepheline and albite from pegmatites. Li, Rb, Cs (detected only in one nepheline), Ba, and Sr were determined, in addition to the major elements. (Pb and Tl were not detected by the spectrographic method used.) Some peculiarities of the mineral structures which are considered to be important for the partition of elements between the coexisting phases are discussed. The importance of vacancies in nephelines, and their probable restriction to the large cavities in the structure, is emphasized. It is pointed out that since low concentrations of Rb, Cs, Pb, and Tl are found in both nepheline and feldspar, they cannot be caused by an unfavourable nepheline structure. They are either a property of the parent magma or are the result of the physical conditions of crystallization (i.e. escaping gas phase). The low concentrations of Sr, and especially of Ba, in nephelines are in contrast to the high contents of these elements in the coexisting feldspars. This is explained by considering the nepheline structure. The unit cell contains eight structural sites for the large cations (Na, K, Ca), and there are eight valencies to satisfy. Six of the sites are smaller than the other two, and preferentially incorporate Na (and Ca) ions. The two larger sites will tend to remain as vacant sites. Those which have to be filled in order to satisfy the charge balance, will preferentially incorporate the large univalent ions (K� and H 3 O�). The higher-charged ions (Ba2� and Sr2�) will pull the anions closer together and reduce the size of the site. They are, therefore, rejected from these positions. When nephelines coexist with feldspar, they will incorporate the maximum amount of Si possible at the temperature of formation. The substitution of Al for Si simultaneously with the entry of a divalent cation (as in feldpspar) is, therefore, not possible except in non-feldspathic, extremely desilicified ... |
|---|